This invention relates to transparent or translucent ceramic compositions and more particularly to ceramic compositions useful for forming metal vapor arc lamp discharge tubes.
Ceramic compositions are used widely in high temperature environments where good chemical and mechanical stability are required. However, the great majority of ceramic compositions are opaque to visible light and therefore cannot be used in a high temperature environment where a transparent window, lamp envelope or the like are required. For example, high pressure sodium lamps are now widely used because they have a higher light-output efficiency as compared to incandescent lamps generally, and they have an efficiency second only to the low pressure sodium lamp. The discharge tubes for the sodium metal vapor arc lamps are constructed from a polycrystalline alumina ceramic or from single crystal sapphire. Inside the presently utilized alumina or sapphire discharge tubes, an amalgam of sodium and mercury metals and some inert gas such as xenon are utilized to generate light. Surrounding the arc tube is an evacuated outer lamp envelope made of glass. A minimum temperature of several hundred degrees is required to vaporize the sodium and is obtained from the light emitting electric arc that also heats the central section of the arc tube to about 1200.degree. C. The material forming the arc tube, in addition to being transparent or trans-lucent, must also be stable for a considerable time period against reaction with the sodium and mercury components within the tube at its operating temperature.
While the polycrystalline alumina ceramics and single crystal sapphire have the requisite degree of transparency for use in such discharge tubes, their use is proven undesirable since they tend to dissociate and vaporize as aluminum metal from the outside of the arc tube, thereby darkening the inner surface of the glass envelope of the lamp. The rate at which this takes place determines the useful lamp life and depends both on the arc tube temperature and the oxygen pressure within the outer envelope. This limitation on lamp life can be overcome, therefore, by placing a lower limit on oxygen pressure or an upper limit on arc tube operating temperature. However, neither approach is completely desirable. In presently utilized arc lamps, a niobium seal is utilized to seal the sodium vapor within the arc tube. However, niobium is permeable to oxygen and any attempt to permit oxygen in the outer envelope eventually leads to sodium loss within the arc tube by oxidation to sodium aluminate. On the other hand, placing an upper limit on arc tube temperature results in an undesirable reduction of light-output efficiency.
Accordingly, it would be highly desirable to provide ceramic compositions which are transparent or translucent to visible light, and which are more stable than pure alumina under the high temperature operating conditions of sodium vapor arc lamps.